Method of producing a magnesium base alloy



United States Patent ()1 3,167,425 Patented Jan. 26, 1965 ice 3,167,425METHOD OF PRGDUCHNG A MAGNESEUM BASE ALLOY James H. T. Petch and PhilipAndrew Fisher, Manchester, England, assignors to Magnesium ElehtronLimited, Swinton, England No Drawing. Filed Apr. 17, 1961, Ser. No.103,220 Claims priority, application Great Britain Apr. 29, 1960 11Claims. (Cl. 75-435) This invention relates to magnesium base alloys ofthe kind containing from 0.1 to 1.0 percent zirconium with or withoutother alloying constituents such as zinc, rare earth metals, thorium,silver, cadmium, lead and other elements which do not cause zirconium toprecipitate from the molten magnesium alloy, and also to alloyscontaining minor amounts of manganese and beryllium commensurate withthe zirconium content.

Among the alloys of this kind which have been commercially produced are(l) wrought alloys containing besides zirconium no other intentionalalloying elements except possibly to 0.005 percent beryllium and 0 to0.4 percent manganese and 0 to 1 percent rare earth metals, (2) wroughtalloys containing besides zirconium about one and one-quarter percentzinc, and (3) cast and wrought alloys containing besides zirconium 2.5to 7.0 percent zinc.

The first mentioned of these two kinds of alloys have not previouslybeen used on a large scale commercially, although they have been studiedscientifically since they comprise the binary system from which morecomplex commercially used alloys such as those containing magnesium,zirconium, zinc, rare earth metals, silver, thorium, etc. have beendeveloped. The binary alloys are becoming of increasing importance inthe nuclear energy industry where they are employed in the Wrought formto sheath the radioactive fuel element and so protect it from possibleattack by the cooling medium. These alloys are particularly suitable forthis purpose in so far that they are characterised by a very small grainsize, a property imparted to them by their zirconium content.

The quality of such protective sheaths must be very high sinceconsiderable distortion may occur in service. The sheath must be able toaccommodate such distortion without cracking since this might lead toexposure of the radioactive material to the coolant. One requirement ofquality in the sheath is that it must be as free as possible fromdiscontinuities such as oxide films, inter metallic compounds or otherdiscrete particles since it would be expected that inclusions of thistype could initiate crack formation during deformation.

During the study of alloys containing magnesium and zirconium, it hasbeen observed that such alloys are characterised by the presence ofinclusions comprising intermetallic compounds of zirconium with elementspresent in the metal as impurities such as iron, silicon, aluminium,manganese and hydrogen. If the alloy is worked these intermetallicparticles may become spread out to form hard stringers in the workpiece,which may have a detrimental effect on its properties and in particularits fatigue endurance. They are therefore highly undesirable in wroughtalloys. In the unworked alloys however, these intermetallic particlesappear to be of little practical significance.

In the case of the magnesium zirconium alloys containing more than 2.5%zinc, a different type of intermetallic inclusion can arise which iscomposed primarily of zirconium and zinc. The zinc-containing alloyshave been used on a considerable scale for purposes requiring hightensile strength and resistance to fatigue at normal temperatures, e.g.for cast aircraft wheels. These large wheels and other castings arecommonly examined radiographically and it is found that clouds of denseintermetallic particles are disclosed on the radiographs of a proportionof such castings. The profusion of the Zn-Zr particles is often such asis likely to affect adversely the properties of even the unworked alloy.Accordingly it is highly desirable to enablelarge castings to beproduced which can be relied on regularly to be free from such particleswhilst maintaining their good properties such as high tensile strengthand low degree of microporosity.

Castings made for subsequent working by plastic deformation (e.g. byextrusion, rolling, forging, etc.) in these alloys with zinc contentsover 2.5% should also. be free of these intermetallic particles, sincethe stringers to which the particles give rise will adversely affect thequality of the product. i

In producing both shaped castings and castings for Working in alloyscontaining magnesium and zirconium of the type covered by the presentapplication, it has always been considered that a temperature of atleast 760 C. should be used to alloy the zirconium into the magnesium inorder to achieve adequately fine grain and high tensile strength.Accordingly the practice has long been to puddle the melt in thepresence of excess zirconium at a temperature of 760 C.-780 C., forwrought stock or 780800 C. for shaped castings and to pour the melt at atemperature of 690 C.-740 C. for wrought stock or at a temperature of760 C.-820 C. for shaped castings. The zirconium may be added in theform of a master salt consisting of a mixture of salts including ahalide of zirconium (eg as described in British patent specification No.715,967) or as a master alloy including metallic zirconium (e.g. asdescribed in British patent specification No. 652,222 or patentapplication No. 857,709 to which U.S. Patent No. 2,970,904 corresponds).The excess zirconium remains at the bottom of the crucible and ispuddled by means of an iron tool such as an iron disc on along iron red,the excess zirconium being stirred by the disc to introduce the maximumamount of zirconium solubly into the alloy.

In attempting to provide material satisfactorily free from inclusions,we have carried out many experiments with a view to eliminating theinclusions from alloys of both the low zinc (or zinc free) and the highzinc types. Thus We have tried the use of filtration using various knowndevices and we have also inserted various types of bafiles in the metalstream with the object of preventing the inclusions from passing intothe castings. We have also tried a number of alloying temperaturesbetween 740 and 800 C. and various zirconium contents in the range(LS-0.7%. Also in the case of the high zinc alloys, tests have beenmadeon various kinds of moulding sands, oxidation inhibitors, mouldtreatment, core composition, mould washes and even variation of alloyingand casting temperatures over a range of 720 to 920 C. By none of thesemethods has it been possible to obtain magnesium-zirconium alloyconsistently free from deleterious zirconium rich particles.

We have found that the Zn-Zr particles are not directly connected withimpurities in the melt but arise when a large body of the alloy is heldmolten at a temperature considerably below the alloying temperature.Thus they will form in large castings where the freezing rate issufliciently slow. They can form in the crucible itself if the pouringtemperature is substantially below the alloying temperature and can thenappear in small as well as large castings.

We have now found that, contrary to accepted belief (J and teaching, itis possible to introduce a full zirconium content into magnesium attemperatures as low as 650 to 700 C. and to obtain very good mechanicalpropertiestherefrom. For this purpose, a master salt containingzirconium fluoride is unsuitable, as also is zirconium metal and it isnecessary to use a master alloy in which the melting point of theentrained salt phase is below 600 C. An example is provided by theinvention of British patent application No. 857,709. When zirconium isintroduced at these low tempenatures, we find that the rate of pick upof impurities by the melt, e.g. iron from the crucible and hydrogen frommoist air, 'fiuxes, etc., is so low that castings poured in a knownmanner from the melt at temperatures not exceeding about 725 C. showvery low incidence of zirconium rich particles.

Moreover, if a high zinc alloy is puddled at these temperatures and castat say 760 C., the level of soluble zirconium achieved in the meltcorresponds to saturation at the low temperature and not at 760 C., sothat insufficient zirconium can separate out on cooling to give rise toextensive ZIk-ZI' particle formation in large castings.

The zirconium may be introduced into the magnesium using a zirconiumalloying substance consisting either of (a) a master alloy made byreduction with magnesium of salt mixture containing zirconium halides,provided the meltingpoint of the entrained salt phase is sufficientlylow, e.g. below 600 C. or of (b) a mixture of zirconium chloride ZrClwith one or more chlorides of the alkali metals and/ or alkaline earthmetals. Thus master alloys in accordance with British Patent No. 652,230and British Patent 857,709 are suitable, whereas salt mixtures inaccordance with British Patents 642,243 and 715,967 and master alloysmade therefrom are unsuitable. Compacts of magnesium and zirconium metalpowder, and other forms of zirconium metal and also unsuitable.

The zirconium master alloy for the purpose of the present inventionpreferably consists of the following 7 British patent specification857,709 viz.:

(1) A salt phase forming 1 to 15 percent by weight of the master alloyand containing MgCl and MgF the latter being from 25 to 40 percent byweight of the MgCl together with alkali metal chloride in quantity byweight from one-third to two-thirds the weight of the MgCl with orwithout BaCl (2) A matrix phase consisting wholly or mainly of magnesiumwith zirconium and with or without permissible elements.

(3) Metallic zirconium embedded in phase (2) and constituting 25 to 45percent by weight of the master alloy.

The master alloy willpreferably be made by the reduction by magnesium ofsalt mixtures containing at least one zirconium halide.

The particular example quoted in specification 857,709 is suitable forthe purpose of the present invention (i.e. a master alloy made by addingmagnesium together with 160 lbs. fluozirconate to a mixture of 364 lbs.MgCI 150 lbs. KCl, and 136 lbs. BaCI If the zirconium alloying substanceis a mixture of chlorides this may be as described in the specificationof British Patent No. 652,230.

According to the present invention, therefore, a methd of producing acasting in a magnesium base alloy containing 0.1 to 1.0 percent byweight of zirconium comprises puddling the alloy at a temperature abovethe liquidus and not exceeding 700 C. in the presence of a zirconiumalloying substance capable of introducing zirconium into magnesium atthese temperatures and casting at a temperature at least equal to thepuddling temperature.

The casting temperature should preferably be slightly above the alloyingtemperature. 7

Generally We prefer to puddle the alloy at a temperature of 660 to 690C. The casting temperature may be 690 to 725 C. for alloys containing upto 2% zinc but may be 700 to 790 C. for alloys containing 2.5 to 7percent zinc.

For an alloy containing 0.4 to 1 percent zirconium and 4 to 5 percentzinc, and used for shaped castings, the alloy may be puddled at 660 and680 C. and cast at a temperature not less than 720 C. For an alloycontaining 0.4 to 1 percent zirconium and 5 to 7 percent zinc, andintended for shaped castings, the alloy may be paddled at a temperatureof 660 to 680 C. and cast at a temperature of from 720 to 790 C.

For producing a casting for subsequent plastic deformation in an alloycontaining 5 to 7 percent zinc the puddling temperature may be 64-0 to680 C. and the casting temperature may be 680 to 725 C. whereas if thealloy contains 2.5 to 4 percent zinc the puddling temperature may be 660to 700 and the casting temperature may be 680 to 725 C.

The process of alloying the zirconium at these low tem peratures may becombined with filtration or similar steps to provide additional controlover the incidence of zirconium rich particles in the casting.

The magnesium zirconium alloy may contain manganese in accordance withBritish Patent No. 806,104. Beryllium may also be incorporated in themelt by any known method applicable to magnesium-zirconium alloys.

The alloy may contain:

Percent by weight Zinc From 0 to 7 Manganese From: 0 to 0.4 Rare earthmetals From 0 to 1.0 Beryllium From 0 to 0.005

with or without other permissable elements known to be compatible withmagnesium-zirconium alloys e.g. as specified in U.S. Patent No.2,788,272.

We claim:

1. A method of producing a casting of a magnesium base alloy containing0.4 to 1.0 percent by weight zirconium which comprises puddling thealloy at a temperature above the liquidus and not exceding 700 C., inthe presence of a zirconium master alloy capable of introducingzirconium into the magnesium at this tempera ture, ringing thetemperature of the alloy to a casting temperature at least equal to thepuddling temperature, and thereupon casting the alloy at said castingtemperature.

2. A method as claimed in claim 1 wherein the alloy is puddled at atemperature of 660 to 690 C., and is cast at a temperature of 690 to 725C.

3. A method as claimed in claim 1, in which the alloy contains at leastone of the following elements in the amounts stated below: 7

Percent by weight 4. A method as claimed in claim 1, for producing acasting for working by plastic deformation in which the alloy containszinc up to 2 percent by weight.

5. A method for producing a casting as claimed in claim 1, in which thealloy contains 2.5-7 percent zinc and the melt is cast at 700 to 790 C.

6. A method for producing a shaped casting according to claim 1 in whichthe alloy contains 0.4 to 1 percent zirconium together with over 4 andup to 7 percent zinc and is puddled at a temperature of from 640 to 680C. and is cast at a temperature of from 720 to 790 C.

7. A method as claimed in claim 1 for producing a casting for working byplastic deformation wherein the alloy contains 2.5 to 4 percent zinc andis puddled at a temperature of 660 to 700 C. and poured at a temperatureof from 680 to 725 C.

8. A method as claimed in claim 1 in which the zirconium master alloy ismade by reduction with magnesium of a salt mixture containing at leastone zirconium halide.

9. A method as claimed in claim 1, wherein said master alloy comprisesthree phases, the first of which is a mixture of halides forming a saltphase and constituting 1 to 15 percent by weight'of the master alloy,said salt phase having a melting point not exceeding 600 C.

10. A method as claimed in claim 9, wherein said second phase comprisesa matrix phase consisting of magnesium with zirconium and with elementswhich do not precipitate zirconium from molten magnesium-zirconiumalloy.

11. A method as claimed in claim 10, wherein said third phase comprisesmetallic zirconium embedded in said second phase.

References Cited in the file of this patent UNITED STATES PATENTS2,497,540 Emley Feb. 14, 1950 2,664,353 Saunders et a1 Dec. 29, 19532,698,230 Doyle Dec. 28, 1954 2,906,619 Roberson et a1 Sept. 29, 19592,919,190 Whitehead et a1 Dec. 29, 1959 FOREIGN PATENTS 652,230 GreatBritain Apr. 18, 1951 134,221 Australia Sept. 21, 1949

1. A METHOD OF PRODUCING A CASTING OF A MAGNESIUM BASE ALLOY CONTAINING0.4 TO 1.0 PERCENT BY WEIGHT ZIRCONIUM WHICH COMPRISES PUDDLING THEALLOY AT A TEMPERATURE ABOVE THE LIQUIDUS AND NOT EXCEDING 700*C., INTHE PRESENCE OF A ZIRCONIUM MASTER ALLOY CAPABLE OF INTRODUCINGZIRCONIUM INTO THE MAGNESIUM AT THIS TEMPERATURE, BRINGING THETEMPERATURE OF THE ALLOY TO A CASTING TEMPERATURE AT LEAST EQUAL TO THEPUDDLING TEMPERATURE, AND THEREUPON CASTING THE ALLOY AT SAID CASTINGTEMPERATURE.